1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter fractions of a fluid sample. More particularly, this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation in order to cause separation of the heavier fraction from the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that overtime globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during the clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical reaction with the gel interface.
Therefore, a need exists for a separator device that (I) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) is able to move into position to form a barrier in less time than conventional methods and devices; (viii) is able to provide a clearer specimen with less cell contamination methods and devices; and (ix) can be used with standard sampling equipment.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly of the present invention includes a rigid outer container, a flexible inner container and a filter assembly for providing communication between the inner and outer containers.
The outer container may be a tube having opposed longitudinal ends and a substantially cylindrical sidewall extending therebetween. Both ends of the tube are substantially closed or closeable. For example, one end of the tube may have a permanent closure extending unitarily from the cylindrical sidewall of the tube. The opposed end of the tube may be substantially open, but may receive a needle pierceable resealable closure. Alternatively, both ends of the tube may be open, and both open ends of the tube may be sealed by elastomeric closures. At least one of the closures of the tube may include a needle pierceable resealable septum.
The inner container may be a flexible collapsible tubular bag formed from a transparent plastic material. The inner container is disposed within the outer container, and in a non-collapsed state may extend substantially between the opposed ends of the outer container. However, the inner container, such as the tubular plastic bag, is selectively collapsible toward one end of the outer container.
The filter assembly comprises a filter that is operative to permit blood serum to pass therethrough. However, the filter will substantially prevent the more dense red blood cells from passing therethrough. The filter assembly further includes a filter support in which the filter is securely retained. The filter support may comprise a cylindrical sidewall having opposed longitudinal ends. An end wall may extend across one longitudinal end of the cylindrical sidewall of the filter support. The end wall includes at least one slit valve formed therein. The slit valve is disposed at a location on the end wall that will substantially register with the filter. For example, the filter may define a substantially thick-walled tube retained by the support of the filter assembly. In this embodiment, the slit valve may define arc sections disposed on portions of the end wall that will register with one end of the tubular filter. In other embodiments, the filter may effectively define a continuous cylindrical plug that is securely engaged within the filter support. In this embodiment, the slit valve can take other configurations, such as a short diametrically aligned slit in the circular end wall.
In all embodiments, the filter assembly is dimensioned to be slidably moveable within the outer container. Additionally, the filter assembly and the flexible inner container define a secure fluid tight connection therebetween. For example, a tubular plastic bag defining the flexible inner container may have portions adjacent the open end disposed between the filter and inner surface areas of the filter support.
In use, a fluid sample enters the assembly by needle. The needle penetrates through the resealable closure and is urged into communication with the interior of the flexible inner container. The sample is then directed into the flexible inner container. The assembly is then placed in a centrifuge such that the filter assembly is at a radially inner position relative to the fluid sample within the flexible inner container. The centrifuge then is operated to place a centrifugal load on the assembly. The centrifugal load causes the more dense phase liquid to move outwardly relative to the axis of rotation of the centrifuge, and simultaneously causes the less dense phase liquid to move into locations closer to the axis of rotation of the centrifuge. The centrifugal load also causes the filter assembly to move away from the axis of rotation of the centrifuge. As a result, the less dense phase liquid is urged into the filter. The centrifugal load also causes the less dense phase liquid to open the slit valve sufficiently for the serum to flow out of the flexible inner container and into the space between the inner and outer containers. The outflow of the less dense phase liquid from the inner container causes the walls of the flexible inner container to collapse gradually, thereby decreasing the volume of the inner container. Simultaneously, there is a corresponding increase in the volume between the inner and outer containers as the less dense phase liquid flows through the filter assembly. After sufficient centrifugation, substantially all of the less dense phase liquid will have passed through the filter assembly. However, the filter prevents a flow of the more dense phase liquid therethrough. As a result, the more dense phase liquid is retained within the inner container, while the less dense phase liquid is retained in the space between the inner and outer containers. Additionally, upon termination of the centrifugal load, the less dense phase liquid disposed in the space between the inner and outer containers will not be subjected to any forces that would cause the less dense phase liquid to migrate back across the filter assembly and into the inner container. As a result, the two phases of the fluid sample may be removed separately from their respective containers and analyzed in a laboratory.
The assembly of the present invention is advantageous over existing separation products that use gel. In particular the assembly of the present invention will not interfere with analytes as compared to gels that may interfere with analytes. Another attribute of the present invention is that the assembly of the present invention will not interfere with therapeutic drug monitoring analytes.
Another notable advantage of the present invention is that fluid specimens are not subjected to low density gel residuals that are at times available in products that use gel.
A further attribute of the present invention is that there is no interference with instrument probes.
Another attribute of the present invention is that samples for blood banking tests are more acceptable than when a gel separator is used.
Additionally, the assembly of the present invention does not require any additional steps or treatment by a medical practitioner, whereby a blood or fluid sample is drawn in the standard fashion, using standard sampling equipment.